Enhance default message logic and setting page layout

[jimsihk]

I would like to propose below changes to the setting pages layout:

1. Add section headers to the setting page to make it clearer on different type of the notifications
2. Display the default values in the settings page
3. Add a help button in the course setting page for the status dropdown to describe the usage in more details

Since the default message values are using different placeholders that site admin could use, I suggest to also revise the logic to align the placeholder values for better maintenance and display.

Note: Sorry that I am not familiar on Spanish so the corresponding translations are not added to the ES translation file...